The structures of semiconductor light-receiving devices are roughly classified into: a front-surface-incident type that receives light incident from a front surface of an epitaxial growth layer on a semiconductor substrate; an end-face-incident type that receives light incident from a side face of the epitaxial growth layer; and a back-surface-incident type that receives light incident from a back surface of a semiconductor substrate.
Generally, a semiconductor light-receiving device can sense light only in a p-n junction region. A larger p-n junction region can more easily adjust the optical axis and provides good mountability. On the other hand, the device capacitance increases with the size of the p-n junction region, which is not advantageous in respect of high-speed response because of increased time constant.
Since back-surface-incident type light-receiving devices can readily be designed with low parasitic capacitance, the p-n joint region can be made accordingly larger, which makes it feasible to achieve both of easy mountability and high-speed operation enabled by reduction of device capacitance. The back-surface-incident type is therefore a commonly used structure for high-speed communications, in particular with a rate of 10 Gbps or more. Moreover, a high quantum efficiency is easier to achieve with a back-surface-incident type light-receiving device because light incident from the backside of the substrate is reflected by electrode metal mirrors back to an absorption layer to be absorbed again. Optical modules using such back-surface-incident type light-receiving devices have been disclosed (see, for example, PTL 1 and PTL 2).